Updates x/net/http2 to git rev 341cd08 for https://golang.org/cl/18576

Change-Id: If5dcb60ac449b798c34fe332ede5ec74e66eb9db
Reviewed-on: https://go-review.googlesource.com/18579Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>